Photoluminescence built-in-test for optical systems

ABSTRACT

A built-in-test capability is provided for determining the integrity of an optical fiber connecting: (a) an optical firing unit having a primary light source emitting a first wavelength, a test light source emitting a second wavelength different from the first wavelength, a mechanism both for coupling light from the light sources to the optical fiber and also for coupling the return light to a detector; and (b) an optically-initiated device which is coupled to a second end of the optical fiber. The apparatus includes a photoluminescent material disposed at a junction of the optically-initiated device and the second end of the optical fiber. In test mode, this photoluminescent material is exposed to the test light source, which results in photoluminescence at a third wavelength. The photoluminescent light travels through the optical fiber to the detector, and when detected indicates optical fiber continuity. The present system can also measure the temperature at the distal end of the optical fiber by detecting changes in the peak wavelength or amplitude associated with the third wavelength as a function of temperature. When the system is used to initiate ordnance, the detector can also detect the initial flash of light produced by the ordnance to provide confirmation that the ordnance has ignited.

RELATED APPLICATION

The present application is a continuation-in-part of the Applicant'sco-pending U.S. patent application Ser. No. 08/739,641, filed on Oct.30, 1996, now U.S. Pat. No. 5,729,012, entitled "PhotoluminescenceBuilt-In-Test For Optically Initiated Systems," which is acontinuation-in-part of U.S. patent application Ser. No. 08/428,377,filed on Apr. 25, 1995, now U.S. Pat. No. 5,572,016, issued on Nov. 5,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fiber optic continuity test systemsand, more particularly, to a single-ended tester capable of detectingdiscontinuities in an optical fiber when operated in a test mode andalso providing confirmation of ordnance ignition.

2. Statement of the Problem

Laser initiated ordnance (LIO) systems are well known in the art andtypically employ a light pulse which is passed along a fiber optic cableand caused to impinge on an energetic material to heat it to ignition.Laser initiated systems are safer than electrical initiation systems inthat the former are not susceptible to inadvertent initiation by staticor stray electromagnetic radiation. In addition to avoiding accidentaloperation, however, ordnance systems are also required to reliablyoperate upon occurrence of a predetermined stimulus.

Therefore, laser initiated systems present two separate, but relatedconcerns. First, the system should provide a means for checking thecontinuity of a firing channel to determine whether the channel ismisaligned, contaminated, mis-mated, severed, crushed or otherwisenonfunctional. Without a test capability, the only available informationrelating to nonfunctionality is that, upon sending a "firing" lightpulse, the ordnance does not initiate. Second, the system should providea means for determining whether the ordnance has ignited after a"firing" light pulse has been sent.

The prior art includes many systems that address only the first concern,i.e., checking the continuity of the fiber optic channel. Fiber opticcontinuity test systems are usually either single ended or dual ended,with singled ended systems being employed in LIO systems because accessto only one end of the fiber is possible. Many single-ended testersutilize optical time domain reflectometry (OTDR). OTDR systems work byfirst transmitting pulses of light into a fiber and then measuring thelight that is reflected back using sophisticated high speed detectionand timing electronics. The time that it takes for the reflected lightto return corresponds to the distance it travels along the fiber. Thisallows the OTDR system to produce a fiber signature. Two types ofreflections occur. Pulse reflections are generated at breaks or jointswhere the light pulse encounters something other than a continuous glasscore. In a typical LIO system, pulse reflections would occur where twosections of fiber-optic cable are connected, and at the interfacebetween the end of the fiber-optic cable and the ordnance. Back scatterreflections are generated uniformly along a fiber as the transmittedpulse travels through the fiber. The back scatter signal provides ameasurement of fiber attenuation. OTDR systems are frequently used forfinding breaks in communication cables which are typically severalkilometers long. One-half meter is considered excellent resolution foran OTDR system. In LIO systems, however, one meter resolution is notacceptable because a break close to the fiber/ordnance interface couldnot be distinguished from the end of the optical fiber by an OTDR system(e.g., a break only a millimeter from the fiber/ordnance interface woulddisable the laser ordnance system but would not be detected by an OTDRsystem). This difficulty is magnified by the fact that thefiber/ordnance interface is a high stress region and is an area wherecracks are likely to form.

Where the resolution of a OTDR system is unacceptable, fiber opticcontinuity systems employing a dichroic mirror have been utilized. U.S.Pat. No. 5,270,537 teaches a continuity test system employing a dichroicfilter (at the fiber/ordnance junction) which reflects light within onewavelength range for continuity test purposes and transmits light withina second wavelength range for ignition purposes. A fiber optic conduithaving a plurality of connectors contained therein connects the lightsources with the ordnance device. The system tests the integrity of theoptical fiber by shining a test laser into the fiber-optic cable. Aportion of the light reflects as it passes each of the plurality ofconnectors. Each of these reflections travels to a detector through thefiber-optic cable and is detected. The majority of the test laser lightwhich remains unreflected continues down the fiber-optic cable and isreflected by the dichroic coating. The reflection of the test laser isalso sent back up the fiber-optic cable and is detected. The system mustbe calibrated to distinguish between the reflections that occur at eachconnector, and the dichroic reflection, i.e., the system must determinethe amount of light that must be reflected by the dichroic mirror toensure there are no breaks in the fiber optic cable. In theory, if thereis a break in the fiber-optic cable, the amount of light which transmitsthrough the break, and is subsequently reflected by the dichroic mirrorwill be at a low level. The detector will detect this low level anddetermine that there is a break in the fiber-optic cable.

U.S. Pat. No. 5,359,192, entitled "Dual-wavelength Low-powerBuilt-in-test For a Laser-initiated Ordnance System" teaches anothercontinuity test system employing a dichroic filter having awavelength-dependent reflectivity. A fiber optic conduit having aplurality of connectors connects the light sources with the ordnancedevice. A dichroic filter is placed at the interface of an ordnancedevice and the optical fiber. The system tests the integrity of theoptical fiber by shining two different wavelengths of test light intothe fiber and detecting the light reflected by the dichroic mirror. Arelative comparison is made of the light reflected by thewavelength-dependent dichroic mirror at the two different wavelengths.Optical continuity is confirmed if more light will be reflected by themirror at one of the wavelengths than the other. This scheme wasdeveloped to overcome the prior art deficiencies of trying todifferentiate the reflections between the conduit connectors and thedichroic mirror reflections because the connector reflections will havea substantially flat optical response within a band encompassing the twowavelengths and therefore do not contribute to the differences in theintensities of the reflected light.

3. Solution to the Problem

Thus, it is desirable to provide a simple and reliable single-endedapparatus for ascertaining fiber optic link continuity from the proximalend of the optical fiber, when operating in a test mode. The presentsystem can also monitor the temperature at the distal end of the opticalfiber. In addition, after the primary light source is fired, thedetector in the present invention can also detect the initial flash oflight from the ordnance to provide positive confirmation that theordnance has ignited. These features are completely absent from theprior art.

SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus is provided fortesting the integrity of an optical fiber connecting: (a) an opticalfiring unit having a primary light source emitting a first wavelength, atest light source emitting a second wavelength different from the firstwavelength, a mechanism for both coupling light from the light sourcesto the optical fiber and also for coupling the return light to adetector (with optional filter); and (b) an optically-initiated device(e.g., ordnance) which is coupled to a second end of the optical fiber.The apparatus includes a photoluminescent material disposed at ajunction of the second end of the optical fiber and theoptically-initiated device. In test mode, this photoluminescent materialis exposed to the test light source, which results in photoluminescenceat a third wavelength. The photoluminescent light travels through theoptical fiber to the detector, and when detected indicates optical fibercontinuity. The present system can also measure the temperature at thedistal end of the optical fiber by detecting changes in the peakwavelength or amplitude associated with the third wavelength as afunction of temperature. When the system is used to initiate ordnance,the detector can also detect the initial flash of light produced by theordnance to provide confirmation that the ordnance has ignited.

A primary object of the present invention is to provide a novelbuilt-in-test apparatus for determining whether there are breaks in afiber-optic link in an optical system.

Another object of the present invention is to provide a novelbuilt-in-test apparatus for determining the type of ordnance device towhich the fiber optic link is connected.

Another object of the present invention is to provide a means forconfirming that the ordnance has initiated.

These and other advantages, features, and objects of the presentinvention will be more readily understood in view of the followingdetailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is a simplified block drawing of a photoluminescencebuilt-in-test structure, in accordance with the present invention;

FIG. 2 is a graph showing the absorption characteristics of nile blue Aperchlorate in polyvinyl butyral;

FIG. 3 is a graph showing the photoluminescence intensity of the nileblue A perchlorate in polyvinyl butyral;

FIG. 4 is a cross-sectional view of a sample quantum wellphotoluminescence structure of the present invention;

FIG. 5 is a graph showing the absorption characteristics of a quantumwell structure;

FIG. 6 is a graph showing the photoluminescence intensity of a quantumwell structure; and

FIG. 7 is a simplified block drawing of a plurality of photoluminescencebuilt-in-test structures.

FIG. 7(a) is a graph showing the photoluminescence intensities of threequantum well structures corresponding to the FIG. 7.

FIG. 8 is a simplified block diagram of another embodiment of thepresent invention in which the detector is used to detect the initialflash of light resulting from ignition of the ordnance.

FIG. 9 is a cross-sectional view of the optical firing unit,beamsplitter, and detector assembly.

FIG. 10 is a graph illustrating the shift in photoluminescenceintensities and wavelengths at various temperatures.

FIG. 11 is a graph corresponding to FIG. 10 showing the peak amplitudeof the photoluminescence as a function of temperature.

FIG. 12 is a graph corresponding to FIG. 10 showing the peak wavelengthof the photoluminescence as a function of temperature.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a photoluminescence built-in-test (PBIT) structure 10adapted for use in an optically-initiated ordnance system. The teststructure 10 includes an optical firing unit 12, a fiber-optic cableassembly 14, and an optically initiated device 16. The optical firingunit 12 has a primary (firing) optical source 12a that emits light at afirst wavelength of light. The primary optical source 12a is preferablya laser that may have an output wavelength and energy covering a broadrange, with the only limitation being that the output energy havesufficient energy to initiate the energetic material 18 withinoptically-initiated device 16. Preferably, the primary optical source12a is a laser that emits light at a wavelength of 850 nm or 980 nm, forexample, and has a power of about 0.1 watts to several watts. Alsohoused in optical firing unit 12 is a secondary (testing) optical source12b that emits light at a second wavelength of light, and a detector12c. The secondary optical source 12b is preferably a laser that mayhave an output energy and wavelength covering a broad range, with theonly limitation being that the output energy does not have sufficientenergy to initiate the energetic material 18 within theoptically-initiated device 16. However, the secondary optical source 12bmust deliver sufficient energy to cause photoluminescense of thephotoluminescent material layer 20 at the distal end 14c of the fiberoptic cable 14a, as will be discussed below. Distinguishing to the testsignal is easily done because of wide difference in the power levelsrequired, and because of the differences in absorption by thephotoluminescent material layer 20 at the first and second wavelengths.For example, commercial diode lasers with low (milli-watt) power ratingsare available in wavelengths of 640 nm, 670 nm, or 720 nm.

The primary laser 12a, secondary laser 12b, and detector 12c are coupledwith the fiber optic cable assembly 14 through well knowninterconnection mechanisms 12d, such as a wavelength divisionmultiplexor or a star splitter/coupler. Another method to interconnectthe lasers 12a and 12b, and detector 12c to the fiber-optic assembly 14is the use of a standard beamsplitter and mirror structure, as taught inabove referenced U.S. Pat. No. 5,270,537 (this patent is herebyincorporated herein in its entirety by reference). Alternatively, apolarized beamsplitter or dichroic splitter/coupler can be used tointerconnect the lasers 12a and 12b, detector 12c. Depending on theintended application of the PBIT structure 10, the primary laser 12a,secondary laser 12b, and detector 12c may be discrete devices, or thesedevices may be monolithically integrated on a single chip (or assembledas a small module as shown in FIG. 9, for example) and coupled to thefiber optic assembly 14 by a standard tapered fiber technique. Thefiber-optic assembly 14 has fiber-optic cables 14a of well known typeand design, and connectors 14b, also of well known type and design thatconnect discrete lengths of fiber optic cables 14a.

In accordance with the present invention, a photoluminescent materiallayer 20 is disposed at a junction 17, between an end 14c of the fiberoptic cable 14a and the ordnance (or energetic material) 18 withinoptically-initiated device 16. The photoluminescent material 20 may bedisposed anywhere within the junction 17 (e.g., on the fiber-end face14c, on either face of, or within a lens 36, or on a face of, or withinthe energetic material 18). By optically-initiated device 16 we mean anyoptically activated device that responds to optical stimulus, (e.g.,energy). Examples of optically-activated devices 16 include: opticalsensors; optical communication system receivers; optically-initiatedordnance systems; fiber amplifier systems; and the like. The presentlypreferred optically-initiated device 16 is an ordnance system, which maybe any one of a variety of devices, such as detonators, initiators,pyrotechnics, and the like.

The optically-initiated device 16 includes a housing 22 having a chargecavity 24 containing energetic material 18, and an optical fiber sealingmeans 26 adapted to secure the optical fiber 14a entering housing 22.The fiber optic cable 14a includes a protective sheath 30, claddingmaterial 32, and a core material 34. To test the integrity of theoptical fiber assembly 14, the test laser 12b emits a beam of light intothe optical fiber assembly 14. The light travels through the opticalfiber core 34, with some of the light being reflected by the connectors14b in the optical fiber assembly, but the vast majority of test laserlight continues through the fiber core 34 and enters the opticalinitiation device 16. The light enters the housing 22 and impinges onthe photoluminescent material 20. The photoluminescent material 20 maybe any of a broad class of materials that absorb at the test laserwavelength, e.g., 670 nm, and photoluminesces upon exposure to the lightof test laser 12b (or soon thereafter). This photoluminescence occurs atdifferent wavelength than that emitted by either primary laser 12a ortest laser 12b. Depending on the intended application of PBIT system 10,it may be preferred that the photoluminescent material 20 besubstantially transparent at the primary laser wavelength, e.g., 850 nm.Transmittance is enhanced by including an anti-reflective coating on thesurface of the PBIT material. Additionally, the light created byphotoluminescent material 20 upon exposure to the light emitted by testlaser 12b should also not be of sufficient energy to heat the energeticmaterial 18 to its auto-ignition temperature.

Optionally, a lens 36, of well known type, may be placed in the junction17 between the fiber 14a and the photoluminescent material 20 to focusthe light exiting the fiber end 14c (i.e., decrease the spot size and toincrease the power density of the light). This focused light augmentsboth the initiation of the energetic material 18 and thephotoluminescence of the photoluminescent material 20. A dichroicmaterial layer 38 may optionally be placed between photoluminescentmaterial 20 and the energetic material 18. The dichroic material 38 issubstantially transparent at the wavelength of light produced by primarylaser 12a and substantially reflective both to the wavelength of lightproduced by test laser 12b and the light produced by thephotoluminescent material 20. The dichroic material 38 helps toconcentrate the amount of test laser light to which the photoluminescentmaterial 20 is exposed and therefore increases the amount ofphotoluminescent light produced. This dichroic material 38 can beintegrated into the PBIT material. Also, when the photoluminescentmaterial 20 photoluminesces, the dichroic material 38 increases theamount of light coupled back into the fiber optic cable 14a through end14c, and therefore increases the amount of photoluminescent lightreaching the detector 12c.

For example, the photoluminescent material 20 can include any of anumber of dye materials contained within a polymer carrier. EastmanKodak Company of Rochester, N.Y., publishes a catalog of opticalproducts that shows the specific absorption and photoluminescencespectra for a plurality of laser dyes. Although not limited by anyspecific list of laser dyes, some examples of useful dye materialsinclude: nile blue A perchlorate (NB); 3-3', diethylthiatricarbocyanineperchlorate (DTTC perchlorate); 3-3'-diethylthiadicarboycyanine iodide(DTDC iodide); and 3-3'-diethylthiatricarbocyanine iodide (DTTC iodide).Each of these must be incorporated into a polymer matrix, with the onlylimitation on the polymer matrix being that, when cured, the polymershould not have substantial absorption in the wavelengths emitted by theprimary laser 12a, the test laser 12b, or the photoluminescence of thephotoluminescent material 20. When a dye is incorporated into a polymer,the wavelength at which it absorbs tends to shift slightly. Althoughthis does not detract from the operability of the present invention, itmust nevertheless be taken into account when assembling PBIT system 10.Examples of suitable polymer carriers include: polyvinyl butyral (PVB);epoxies such as "EP30-1," made by Master Bond Epoxy, Hackensack, N.J.and "Epo-Tek 310" made by Epoxy Technology, Inc., Billerica, Mass.;Norland Optical Adhesive 61, made by Norland Products, Inc.,Newbrunswick, N.J.; and Lens Bond Optical Cement Type SK-9, made bySummers Optical, of Fort Washington, Pa. The laser dyes are added to thepolymer until a saturated solution is obtained. The polymer is thenspread into a thin film and cured.

One preferred photoluminescent material 20 comprises a NB laser dyecontained within a PVB polymer carrier at a concentration of 7×10¹⁵molecules per squared centimeter. Incorporation of NB in PVB does notsubstantially shift the absorbency of NB. FIG. 2 shows the absorbencycharacteristics of an NB/PVB film over a range of wavelengths. As shown,the absorbency is maximized at approximately 640 nm (the test laserwavelength) and a minimum absorbency at approximately 850 nm (theprimary laser wavelength). FIG. 3 shows the output power thephotoluminescence light produced by this same NB/PVB film. As shown, thephotoluminescence peak is maximized at approximately 672 nm which isdifferent than the wavelength emitted by either the primary laser 12a orthe test laser 12b, and is of sufficient power to travel through thefiber optic cable assembly to the detector 12c. It should be understoodthat there are a wide range of capable laser dye materials for theapplication of the present invention and that all such materials arewithin the scope of the present invention.

Polymer carriers for laser dye materials have limited usefulness inspace applications due to outgassing of the solvents within the polymersolution prior to curing, and other well known problems with polymeroperation at very low pressures and temperatures. Therefore, for spaceor other applications, a presently preferred photoluminescent material20 utilizes semiconductor structures. For example, direct band gapsemiconductors have photoluminescent properties, and thephotoluminescence peak will shift depending on the width of the bandgap. Although the photoluminescence peak can shift over a broad range,to achieve a photoluminescence peak of approximately 747 nm, the bandgap of the semiconductor should be approximately 1.6 eV. At present, thepreferred semiconductor structure is a quantum well structure. With aquantum well structure the photoluminescence peak can be tailored with ahigh degree of precision and the intensity of the photoluminescencelight is typically higher than that obtainable with laser dyes. Adetailed discussion of quantum well technology can be found in C.Weisbuch, "Quantum Semiconductor Structures" (Academic Press, Inc.,1990). Quantum well photoluminescent materials can be made using manydifferent kinds of semiconductors, which will be known to those skilledin the art. The choice of materials and layer structure is determined bythe wavelength region to be absorbed and the wavelength ofphotoluminescence to be emitted. It should be understood that all suchsemiconductor systems and all such structures are within the scope ofthe present invention.

FIG. 4 shows a sample structure of a quantum-well-photoluminescentmaterial (QWPM) 100 according to the present invention. QWPM 100 isprepared, or grown, in the following manner. Growth starts with acommercially produced substrate 110 having a mechanically and chemicallypolished surface 110a. Then, a 0.5 micron thick buffer layer of galliumarsenide (GaAs) 112 is grown to cover any damage to the crystal latticestructure caused by the polishing process. Next, a 1 micron thickseparation layer 114 of aluminum gallium arsenide (AlGaAs) is disposedthereon. This separation layer has an aluminum content on the order of95-100 percent. The separation layer 114 is used to facilitate layerremoval of the substrate 110 and buffer layer 112 from the structure100. The next layer grown is a 0.12 micron thick GaAs etch stop layer116 which is insensitive to the chemical etchants used to remove theseparation layer 114. If desired, an optional dichroic material 38, asdescribed in detail above, can be integrated into the QWPM 100 as layer118. The dichroic layer 118 includes a plurality of alternating andjuxtaposed layers of GaAs and AlGaAs built up to a desired thickness.Each layer of GaAs and AlGaAs is approximately 52 nm and although thethickness may vary, depending on the wavelength of light to be reflectedand the amount of reflectivity needed, a typical thickness of layer 118is approximately 1 micron. Two 0.25 micron layers of AlGaAs cladding(120 and 124) are disposed on either side of quantum well structure 122,and are chosen to be optically transparent to the light emitted by theprimary laser 12a, the test laser 12b, and the photoluminescent peakemitted by the quantum well region. The purpose of cladding layers 120and 124 is to block the escape of carriers (electrons and holes)generated by absorption in quantum well region 122, and such confinementhelps maximize the emitted photoluminescence intensity.

The quantum well structure 122 consist of a series of alternating andjuxtaposed layers of AlGaAs barrier layers and GaAs wells. The thicknessof each barrier layer is approximately 5 nm and the thickness of eachwell is approximately 10 nm. The carriers generated by opticalabsorption fall into the wells and then recombine (from well-definedquantized energy levels) to emit photoluminescent light. By choosing thethickness and composition of the barriers and wells, both the opticalabsorption wavelength region and optical emission (photoluminescence)can be tailored. On top of the QWPM 100 is a thin GaAs cap 126 toprotect the top AlGaAs cladding layer 124 from moisture.

The wavelength of the photoluminescent light produced by the quantumwell structure may shift with changes in temperature. This effect can besubstantially reduced by creating a series of quantum wells havingslightly different photoluminescent wavelengths that are staggered overa predetermined range. In this configuration, at least one of thequantum well structures will photoluminesce at the wavelength monitoredby the detector 12c over the entire temperature range.

FIG. 5 shows the absorbency characteristics of a quantum well structure(100 in FIG. 4) for various wavelengths of light. As shown, the materialtransmits light at the wavelength produced by primary laser 12a (e.g.,850 nm) and absorbs at the wavelength produced by test laser 12b (e.g.,640 nm). FIG. 6 shows the output photoluminescence power produced by thesame quantum well structure. The maximum power output occurs atapproximately 747 nm which is different than the wavelength emitted byeither the primary laser 12a or test laser 12b, and is sufficient totravel through fiber optic cable assembly to the detector 12c. It shouldbe understood that those skilled in the art can vary the quantum wellstructure to tailor the absorbency and transmittance for a particularapplication, and that all such structures are within the scope of thepresent invention.

Referring again to FIG. 1, the photoluminescent material 20photoluminesces when exposed to test laser 12b. As the photoluminescedlight travels back through the fiber optic assembly 14, it is coupled tothe detector 12c which is configured to measure only that wavelength oflight. As stated above, the photoluminescent light has a differentwavelength than the light emitted by either primary laser 12a or testlaser 12b. Thus, if the detector 12c is configured to only measure theknown wavelength of light produced by the photoluminescence of material20, the detector 12c will not measure any of the reflections of lightfrom the primary laser 12a and the test laser 12b caused byinterconnections 14b in fiber-optic assembly 14. This is accomplished bypositioning a filter assembly, of well known type, within the detector12c such that only the photoluminescence wavelength passes through to bedetected. This capability simplifies the overall PBIT system 10 in thatno high speed electronics are needed to calculate the time of thesereflections, nor does the detector 12c need to distinguish betweenreflections of the same wavelength but having slightly differentintensities. The detector 12c need only look for the wavelength producedby the photoluminescent material 20, and if detected, continuity of thefiber-optic cable assembly 14 is confirmed. If a break is present withinthe assembly 14, a substantial portion of the light from test laser 12bwill be reflected and the small amount of light impinging on thephotoluminescent material 20 will cause a photoluminescent peak of verylow intensity. This peak intensity will travel back through the fiberoptic assembly, will be reflected by the break, and the amount reachingthe detector will be of sufficiently low intensity, i.e., below somepredetermined threshold, so as to indicate a break.

Although it is presently preferred that the primary laser 12a and testlaser 12b have distinct wavelength ranges, such that thephotoluminescent material 20 absorbs test wavelength and does not absorbprimary wavelengths, it should be understood that the primary laser 12aand test laser 12b may emit the same wavelength of light. In such anembodiment, primary laser 12a and test laser 12b can be combined intoone laser 12a with two power settings (high and low). The wavelength oflight emitted by laser 12a corresponds to the absorption peak ofphotoluminescent material 20. Thus, at low power settings, the lightemitted by laser 12a is absorbed by the photoluminescent material 20,which will photoluminescence and the photoluminescent light will travelback through the fiber optic assembly and will be detected. As long asthis low power setting does not have sufficient energy to combust theenergetic material 18, there is no danger of ignition during a testpulse. Since the high power setting will also be absorbed byphotoluminescent material 20, there must be sufficient power to passenough energy through the photoluminescent material 20 to combust theenergetic material 18. This can be accomplished either by having theenergy not absorbed by the photoluminescent material 20 of sufficientpower to initiate combustion of the energetic material 18, or by havingenough power to essentially vaporize the photoluminescent material 20and then pass energy through to combust the energetic material 18.Additionally, this could be accomplished by photo-bleaching of thephotoluminescent material, because some photoluminescent materialsbecome substantially transparent after being exposed light above acertain power level.

While the photoluminescent material 20 is described in detail herein asit relates to an optically initiated device, it should be understoodthat the photoluminescence built-in-test structure of the presentinvention can be used with other optical systems where a known returnsignal is desired from a certain location. As stated above, theoptically initiated devices of the present invention can be anyoptically activated device which responds to optical stimulus.

In accordance with yet another aspect of the present invention, avariety of different photoluminescent materials 20, each of whichphotoluminescence at a distinct and particularly different wavelength,can be designated to, and paired with, a distinct type of ordnancedevice. As stated above, a typical optically-initiated ordnance systemmay have a multitude of distinct ordnance devices for various users. Forexample, one ordnance device may be used for rocket ignition, anotherfor staging, and another for flight termination. Each of these ordnancesmay be paired with a different particular photoluminescent material(i.e., having a different particular photoluminescent peak) todistinguish each ordnance device during fiber-continuity tests. Such asystem could be used to ensure that each and every use (e.g., stagingand ignition) is connected to the proper ordnance.

Referring now to FIG. 7, an optical firing unit 12 has a primary opticalsource (laser) 12a, a test optical source (laser) 12b, coupling means12d and a detector 12c capable of filtering out and measuring a varietyof wavelengths of light. Optical firing unit 12 is connected to aplurality of optically-initiated devices, e.g., 16a, 16b and 16c, by arespective fiber optic cable assembly (14i, 14ii, and 14iii). Each ofthe optically-initiated devices has a respective photoluminescentmaterial (20a, 20b, 20c) that photoluminesces at a different distinctwavelength, as illustrated in FIG. 7(a). In operation, a common testlaser 12b would fire, directing a first wavelength of light down allfiber optic cables (14i, 14ii, and 14iii) to all of theoptically-initiated devices (16a, 16b and 16c). The detector 12c wouldbe set by controlling electronics 12e to measure only the knownwavelength of, for example, the first photoluminescent material 20a.Since absorption in the photoluminescent material is broad band, and thephotoluminescent light is narrow band, a single test laser 12b, can beused which has a wavelength within the broadband absorption of all ofphotoluminescent materials (20a, 20b and 20c), and each photoluminescentlight wavelength, being of narrow band, will be separate and distinct(for optically-initiated device identification), as shown in FIG. 7(a).Once the detector 12c confirms the continuity of the fiber-optic cable14i, the test laser 12b would fire again, but the controllingelectronics 12e would reset the detector 12c to measure only thedifferent, second wavelength emitted from the second photoluminescentmaterial 20b. If the photoluminescent peak of the secondphotoluminescent material 20b is not detected, the controllingelectronics 12e could drive detector 12c to scan the wavelength rangesfor photoluminescent materials 20a and 20c to determine if the wrongoptically initiated device was installed. This process would be repeateduntil the continuity of all fiber optic cable assemblies were confirmedand, additionally, it is confirmed that the proper optically-initiateddevice is placed in the correct location of the overall system.

Initiation Monitor

In aerospace applications, redundant optical initiation systems arecommonly use to improve reliability. These optical initiation systemsare triggered in parallel to help ensure that ordnance events occur whendesired. At present, there is no way to determine whether each of theoptical initiation systems functioned properly. Monitoring the endresult of the ordnance event only indicates whether at least one of theredundant optical initiation systems worked, but not whether they allworked. For example, with two parallel optical initiation systems, thereis no way to tell whether the systems have had up to a 50% failure rate.

FIG. 8 shows another embodiment in which the detector 12c is also usedto detect light resulting from combustion of the energetic material 18in the ordnance that is transmitted back through the fiber optic cable14. This permits the detector 12c to provide immediate confirmation thatthe primary laser 12a has successfully ignited the ordnance. Ignition istypically accompanied by a flash of light from the ordnance. A portionof this initial flash is captured by the fiber optic cable and travelsback to the detector 12c. The flash is usually white light, or at leastsufficiently broad-spectrum, so that a significant portion of the lightfrom the flash is detected by the detector 12c. The controllingelectronics 12e can be programmed to monitor the output of the detector12c during a predetermined time period after the primary laser 12a hasbeen fired to determine success or failure of the optical initiationsystem. As shown in FIG. 8, this information can be transmitted by atransmitter 81 associated with the control electronics 12e to a remotereceiver 82 by telemetry or wire, even in a destruct scenario providedthe control electronics and transmitter hardware are fast enough.

FIG. 9 provides further details of the preferred embodiment of theoptical firing unit 12, including the primary laser 12a, test laser 12b,detector 12c, and beamsplitter 12d. The primary laser 12a and thedetector 12c can be housed in a single assembly as shown in FIG. 9. Thebeamsplitter 12d directs light from either the primary laser 12a or testlaser 12b into the proximal end of the optical fiber 14. Any lightreturning through the optical fiber 14 (i.e., luminescent light, orlight resulting from combustion of the ordnance) is directed into thebeamsplitter 12d and a substantial portion of this light falls onto thedetector 12c, as previously discussed. A series of lenses 91, 92, and 93focus the light emitted by the light sources 12a and 12b into the end ofthe optical fiber 13.

Built-In-Test Interlock.

Lasers are classified as either class I (safe), II, III, or IV, based ontheir eye and skin hazard. LIO systems typically use class III or IVlasers that require a myriad of laser safety precautions, and mayinclude expensive facility modifications. Lasers may be declassified toclass I if the light is completely contained, therefore requiring nosafety precautions. The BIT system described above can be used as aninterlock mechanism for the primary laser 12a to permit declassificationof the LIO system to class I. The BIT system is capable of detectingwhether ordnance, a simulator, or other device is properly attached tothe distal end of the fiber optic cable, thus containing the laserlight. If the system does not pass the BIT, the controlling electronics12e can be programmed to not allow the primary laser 12a to fire. Ifused in an aerospace application, the interlock can be disabled beforeflight to eliminate concerns about introducing a possible means forcausing the ordnance to fail to fire.

Thermal Insulation.

The LIO system must locally heat the ordnance material to the point ofauto-ignition. The dominant thermal conduction path to overcome is backthrough the optical element introducing the light. A thermal insulatorbetween the fiber optic cable and the ordnance significantly reduces theamount of energy required to cause initiation of the ordnance. In thepresent invention, the photoluminescent material provides a significantinsulating effect that reduces thermal conduction back through the fiberoptic cable and thereby accelerates ignition of the ordnance.

Temperature Effects on Semiconductor Luminescence.

The previous discussions describe the semiconductor material applied tothe distal end of a fiber optic for built-in-test purposes. The materialis used to generate a unique wavelength of luminescence coupled to thefiber once the material has been optically pumped through the samefiber. Such a configuration may also be used as a remote temperaturesensor at the distal end of a fiber optic based on semiconductorluminescence intensity and wavelength changes with temperature, each ofwhich are readily discerned with proper detection.

The properties of semiconductor materials are functions of temperature.Some of the properties vary more with temperature than others (i.e. arestronger functions of temperature). The properties that are of interestto this discussion include lattice constant, bandgap, minority carrierlifetime, and free carrier distribution. These particular properties aremajor factors in determining the behavior of free electrons in thematerial's conduction band, and holes (the absence of electrons) in itsvalence band. This behavior, in turn, directly determines theluminescent properties of the material.

Recombination of electrons with holes occurs in semiconductors viaradiative and nonradiative processes. Luminescence is the term used todescribe the event whereby an electron and hole recombine and emit aphoton (radiative process). If the radiative process involves anelectron at the bottom of the material's conduction band (E_(c))directly combining with a hole at the top of the valence band (E_(v)),then a photon of energy equal to the semiconductor's bandgap (Eg=Ec-Ev)is emitted. Measurement of luminescence emitted from a semiconductorusually shows a large peak centered around the wavelength associatedwith this energy. The width of the peak is due to thermal smearing ofthe bands and allowed transitions of electrons above and slightly belowthe conduction band edge to available hole sites below and slightlyabove the valence band edge.

In general, the bandgap of a semiconductor material will decrease withincreasing temperature. The reduced bandgap manifests itself by theshifting of the luminescence peak to longer wavelengths, i.e., lowerenergy, as temperature increases (see FIGS. 10-12). This is a directresult of thermal expansion. The atoms in the semiconductor's crystallattice move farther apart with increasing temperature, changing thecoupling between adjacent atoms and thus, altering the band structure.The shift is a well-behaved function in common III-V semiconductorcompounds of the form: ##EQU1## and can be utilized to determine thetemperature of the material. In this equation, T is the temperature indegrees Kelvin of the material and A, B, and C are constants. Forexample, in bulk GaAs, A=1.519, B=5.405×10⁻⁴, and C=204. Knowing whatmaterial is luminescing and measuring the wavelength of the luminescencepeak can result in an indication of the material's temperature.

A side effect of temperature on the luminescence is to alter the peakwidth and height, as shown in FIGS. 10 and 11. As the temperatureincreases, the peak height drops and the width broadens. Severaltemperature dependent properties of the material take part in thesephenomena but the primary ones are free carrier distribution andminority carrier lifetime.

Free carriers occupy states within the band structure of the material.The number of free carriers available at any given energy level followsa Fermi-Dirac distribution function: ##EQU2## where again, T istemperature, k is the Boltzmann constant, and E_(F) is the Fermi levelin the material. The Fermi level energy is determined by the bandstructure and the kind (electron and hole) and number of free carriers.An increase in the temperature of the material causes the free carrierdistribution to spread over a wider range of energies. This means thatthe free carriers available to emit luminescence are spread over a widerrange of allowed states. Photons are emitted from recombinationsoccurring over a broader range of energies, thus widening theluminescence peak. In addition, spreading the carriers over a broaderrange of energies effectively reduces the number available for band edgeto band edge recombination, thereby reducing the peak height.

While the peak height may be reduced and the width broadened, generally,within a reasonable temperature range, the total power emitted (i.e.,the area under the luminescence peak curve) will stay constant. However,as the temperature continues to increase, another effect begins tomanifest itself. For a material to have efficient luminescence,radiative processes must dominate over competing nonradiative processes.Increasing the temperature of the material tends to increase the amountof time a free carrier occupies a given site. This increase in timetends to increase the probability that a free carrier will recombine viaa nonradiative path in the material, thus reducing the totalluminescence intensity from the material.

The effects mentioned above are basically true whether the materialstructure is a quantum well design or just pure bulk material.Temperature effects on the luminescent properties of quantum wells aloneare, in the case of the present discussion, second and third ordereffects. The height above the bottom of the well of the first allowedstate does not change appreciably with temperature. However, the bottomof the well is set by the band edge which, as discussed above, doeschange (bulk effect). The spreading of the free carrier distributioneffectively acts much the same as with bulk material, spreading carriersinto other allowed energy states either in the well or above, wideningthe luminescent peak.

The wavelength and/or amplitude of the photoluminescent peak can bemeasured by means of the detector 12c and converted to a temperaturemeasurement using calibration curves as shown in FIGS. 11 or 12. Thepreceding discussion has addressed the use of semiconductor materialsand quantum well structures at the distal end of the optical fiber toproduce photolumenescence. However, it should be noted that any of awide variety of photoluminescent materials could be substituted.

While the invention is described herein in some detail, manymodifications and variations will become apparent to those skilled inthe art. It is our intent to be limited only by the scope of theappending claims, and not by the specific details or instrumentalitiespresent herein by way of description of the preferred embodiments.

We claim:
 1. An apparatus for measuring the temperature at the distal end of an optical fiber, said apparatus comprising:an optical fiber having a first end and a second end; a light source for selectively transmitting light energy within a predetermined wavelength range into said first end of said optical fiber; a photoluminescent material disposed at said second end of optical fiber, said photoluminescent material being photoluminescent when exposed to said light source wavelength range at a wavelength determined as a function of temperature, said photoluminescent light having a wavelength different than said light source wavelength range, said photoluminescent light being coupled into said second end of said optical fiber for passage through said optical fiber to said first end; and a detector coupled to said first end of said optical fiber for responding to said photoluminescent light to indicate temperature at said second end of said optical fiber.
 2. The apparatus of claim 1 wherein said detector measures said temperature as a function of the wavelength of said photoluminescent light.
 3. The apparatus of claim 1 wherein said detector measures said temperature as a function to the amplitude of said photoluminescent light.
 4. The apparatus of claim 1 wherein said photoluminescent material comprises a semiconductor for transmitting light in a first wavelength range and photoluminescing where exposed to a second wavelength range.
 5. The apparatus of claim 4 wherein said semiconductor includes a quantum well structure.
 6. The apparatus of claim 5 wherein said quantum well structure comprises a gallium-arsenide coating applied to said second end of said optical fiber.
 7. The apparatus of claim 5 wherein said quantum well structure incorporates a dichroic mirror for transmitting light in a first wavelength range, and reflecting light in a second wavelength range.
 8. An apparatus for testing the integrity of an optical fiber in a test mode and for initiating ordnance coupled to said fiber in an initiation mode, said apparatus comprising:an optical fiber having a first end and a second end; a light source for selectively transmitting light energy within a first wavelength range into said first end of said optical fiber in said test mode, and for transmitting light energy within a second wavelength range into said first end of said optical fiber in said initiation mode; optically-initiated ordnance coupled to said second end of said optical fiber which, when exposed to light in said first wavelength range, is heated to an auto-ignition temperature and then emits initiation light energy that is coupled into said second end of said optical fiber for passage through said optical fiber to said first end; a photoluminescent material disposed at a junction of said ordnance and said second end of said optical fiber that is substantially transparent to said first wavelength range, said photoluminescent material being photoluminescent when exposed to said second wavelength range at a wavelength determined as a function of temperature, said photoluminescent light having a wavelength different from either said first or said second wavelength range, said photoluminescent light being coupled into said second end of said optical fiber for passage through said optical fiber to said first end; and a detector coupled to said first end of said optical fiber for responding to said photoluminescent light to indicate continuity of said optical fiber in said test mode and to indicate temperature at said second end of said optical fiber.
 9. The apparatus of claim 8 wherein said detector measures said temperature as a function of the wavelength of said photoluminescent light.
 10. The apparatus of claim 8 wherein said detector measures said temperature as a function to the amplitude of said photoluminescent light.
 11. The apparatus of claim 8 further comprising a dichroic mirror disposed on a surface of said photoluminescent material remote from said second end of said optical fiber, wherein said dichroic mirror is transparent to light in said first wavelength range, and is reflective to both light in said second wavelength range and said photoluminescent light.
 12. The apparatus of claim 8 wherein said light source comprises:a primary light source for transmitting light energy within said first wavelength range; a test light source for transmitting light energy within said second wavelength range; and a beamsplitter for directing said light energy from said primary light source and said test light source into said first end of said optical fiber.
 13. The apparatus of claim 8 wherein said photoluminescent material comprises a semiconductor for transmitting light in said first wavelength range and photoluminescing where exposed to said second wavelength range.
 14. The apparatus of claim 13 wherein said semiconductor includes a quantum well structure.
 15. The apparatus of claim 14 wherein said quantum well structure incorporates a dichroic mirror for transmitting light in said first wavelength range, and reflecting light in said second wavelength range and said photoluminescent light.
 16. An apparatus for testing the integrity of an optical fiber in a test mode and for confirming initiation of ordnance coupled to said fiber in an initiation mode, said apparatus comprising:an optical fiber having a first end and a second end; a primary light source for transmitting light energy within a first wavelength range in said initiation mode; a test light source for transmitting light energy within a second wavelength range in said test mode; means for directing light from said primary light source and said test light source into said first end of said optical fiber; optically-initiated ordnance coupled to said second end of said optical fiber which, when exposed to light in said first wavelength range, is heated to an auto-ignition temperature and then emits initiation light energy that is coupled into said second end of said optical fiber for passage through said optical fiber to said first end; a photoluminescent material disposed at a junction of said ordnance and said second end of said optical fiber that is substantially transparent to said first wavelength range and at least a portion of said initiation light, said photoluminescent material being photoluminescent when exposed to said second wavelength range at a wavelength determined as a function of temperature, said photoluminescent light having a wavelength different from either said first or said second wavelength range, said photoluminescent light being coupled into said second end of said optical fiber for passage through said optical fiber to said first end; and a detector coupled to said first end of said optical fiber for responding to said photoluminescent light to indicate continuity of said optical fiber in said test mode and to indicate temperature at said second end of said optical fiber, and for responding to said initiation light to confirm initiation of said ordnance in said initiation mode.
 17. The apparatus of claim 16 wherein said detector measures said temperature as a function of the wavelength of said photoluminescent light.
 18. The apparatus of claim 16 wherein said detector measures said temperature as a function to the amplitude of said photoluminescent light.
 19. The apparatus of claim 16 further comprising a dichroic mirror disposed on a surface of said photoluminescent material remote from said second end of said optical fiber, wherein said dichroic mirror is transparent to both light in said first wavelength range and at least a portion of said initiation light, and is reflective to both light in said second wavelength range and said photoluminescent light.
 20. The apparatus of claim 16 wherein said photoluminescent material comprises a semiconductor for transmitting light in said first wavelength range and photoluminescing where exposed to said second wavelength range.
 21. The apparatus of claim 20 wherein said semiconductor includes a quantum well structure. 